1. Field of the Invention
The invention relates to an emergency standby power system for automatic transfer from utility power to a battery powered inverter when the normal source of utility power is interrupted to a given load.
2. Description of the Prior Art
The prior art generally discloses systems wherein a battery driven inverter is automatically switched into a circuit when the normal line voltage fails. The Rich U.S. Pat. No. 3,818,272 relates to a standby power system employing SCR inverters in which the load at the output of the inverter is automatically removed in the event a load short circuit occurs, and in which high surge current requirements on the inverter by the load are limited to avoid commutation failure in the SCR inverter. The surge and short circuit detection circuit automatically connect the load back to the inverter at a predetermined time, once having made an initial connection. Miller et al U.S. Pat. No. 3,925,772 relates to a standby power supply, in which a battery sensing circuit is included for producing a control signal when the charge of the battery falls below some predetermined level to replenish the D.C. source.
Circuits to provide proper commutation and operation generally are also known. For example, Eastop U.S. Pat. No. 3,657,634 applies trigger pulses to the SCR's before voltage zero of the natural resonant frequency of the load to ensure adequate buildup of current in the SCR's. Kondo et al and Pelley et al U.S. Pat. Nos. 3,534,243 and 3,599,078 pre-charge the commutating capacitor to ensure proper commutation. Larsen U.S. Pat. No. 3,350,625 uses a time delay relay which delays conduction of the silicon controlled rectifiers and application of supply voltage to a trigger signal generating circuit for a predetermined time interval after closing the main circuit breaker or switch, to prevent erratic operation which might otherwise result from voltage transients produced during closing of the breaker or switch.